Role of Structural Defects in the Water Adsorption Properties ofMOF-801Jongwon Choi,† Li-Chiang Lin,‡,* and Jeffrey C. Grossman†,*†Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, UnitedStates‡William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210,United States

*S Supporting Information

ABSTRACT: The nanoporous and tunable nature of metal−organic frameworks (MOFs) has made them promising adsorbentsfor water adsorption applications such as water harvesting andadsorptive heat pumps. In these applications, water adsorptionproperties in MOFs play a crucial role. However, understandingtheir structural defects and how defects influence adsorptionthermodynamics remains limited to date. In this work, byemploying Monte Carlo techniques and first-principle densityfunctional theory calculations, we investigate the effect of defects onthe water adsorption properties in MOF-801 structures at an atomiclevel. Our calculations show that the adsorption isotherm in perfectMOF-801 (without defects) greatly deviates from that measuredexperimentally. With the introduction of defects with a high density,a reasonably good agreement can be achieved, suggesting that a high defect density in MOF-801 may be responsible for itshydrophilic adsorptive behaviors. Further, water adsorption properties in MOF-801 structures are found to depend on the spatialconfiguration of defects, and water condensation in nanoporous MOF-801 is identified to occur preferentially along the ⟨110⟩direction. Detailed structural characteristics (accessible volume, etc.) of MOF-801 structures and the adsorption energetics ofwater in the frameworks are also studied and correlated with the computed adsorption isotherms. Our findings reveal importantinsights into the role of defects, offering a microscopic picture to help facilitate the rational design of better MOFs for wateradsorption applications.

■ INTRODUCTIONMetal−organic frameworks (MOFs) are an emerging class ofcrystalline nanoporous materials that have received consid-erable attention over the past decade.1−4 Their controllablepore structures,5 exceptionally high surface area,6,7 and decentthermal and chemical robustness5 have made them promisingcandidates as adsorbents for a variety of energy- andenvironment-related applications.5−10 Specifically, among awide range of possible guest molecules that have been studiedfor their adsorption properties in MOFs, water adsorption inMOFs has received particular attention recently for water-harvesting and heat transformation applications.11−15 A recentexperimental study reported that 1 kg of MOF-801 is capable ofcapturing 2.8 L of water daily from ambient air at a low relativehumidity level (∼20%) without any additional energy input.11MOFs have also been proposed as adsorbents for heattransformation applications, that is, heat pumps, thermalstorage, and/or refrigeration12−15 because of the large heatsof adsorption together with high adsorption capacity.16 Asattention around water adsorption applications using MOFscontinues to grow, an improved atomic-level understandingregarding the water adsorption properties in MOFs is of critical

importance toward the continued rational design of MOFs withenhanced performance.One important aspect of MOFs is their structural defects,

which can be potentially exploited and designed to engineermaterials properties and therefore enhance their performance ina given application.17−19 Although using MOFs as wateradsorbents has drawn increasing attention, understanding theeffect of structural defects on their water adsorption propertiesremains limited. As an example, one of the most heavily studiedwater-stable MOFs, UiO-66, is prone to possess a high defectdensity of ∼1/12 (i.e., two missing linkers per unit cell).20−26Although the presence of defects may not be ideal for thethermal and mechanical stability of MOFs,21,24 it has beendemonstrated both theoretically27 and experimentally26,28 thatdefects can increase the hydrophilicity of MOFs, leading toenhanced water adsorption properties. Yet, it remains generallyunclear how the spatial configuration of defects will affect thewater adsorption properties. In addition, little is known at the

Received: January 2, 2018Revised: February 18, 2018Published: February 21, 2018

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCChttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpcc.8b00014http://dx.doi.org/10.1021/acs.jpcc.8b00014

microscopic level about the water adsorption process insideMOFs that contain defects. Accordingly, we aim to shed lighton the effects of configuration and density of defects on thewater adsorption properties. Specifically, we focus on MOF-801,11,29,30 another water-stable MOF structure, that has beenstudied previously for its promising water-harvesting ability asintroduced above. MOF-801 is a zirconium-based MOF with achemical composition of Zr6O4(OH)4(fumarate)6 (i.e., in theabsence of defects, denoted as perfect MOF-801 structures).MOF-801 may also possess defects as discussed in the work ofFurukawa et al.30 This is also reflected on the results of theelementary analysis, where the carbon and hydrogen elementpercentage of the synthesized samples after activation wasfound to be lower than that of perfect MOF-801 structures.30

In this study, state-of-the-art Monte Carlo simulations in thegrand canonical and canonical ensembles are employed to showthat the water adsorption in MOFs occurs primarily near defectsites and with a growing number of defects, the MOF becomesmore hydrophilic. This observation is supported by thecalculated adsorption energetics (intermolecular interactions)of water with defective MOF structures. At a defect density of1/6 (four missing linkers per unit cell in MOF-801), thesimulated water adsorption isotherm resembles the exper-imentally reported30 one reasonably well. Furthermore, adetailed investigation of the spatial arrangement of defectsindicates its profound effect on the water adsorption behavior,and preferential water adsorption sites are observed in the⟨110⟩ directions of the MOF-801 crystal. Depending on thedefect configuration, the density of the adsorbed water phase inMOF-801 can vary considerably, which, in turn, affects themaximum adsorption capacity of the MOF structure.

■ COMPUTATIONAL DETAILSIn this study, a perfect MOF-801 crystal and five illustrativeMOF-801 structures with different defect densities and/orconfigurations as shown in Figure 1 were systematicallyinvestigated. The particular type of defects focused herein isthe missing-linker defects. We considered defect densitiesvarying from 1/24, 1/12, to 1/6, corresponding to 1, 2, and 4

missing linkers per unit cell, respectively (denoted also,respectively, as def1, def2, and def4). To obtain insights intothe effects of spatial defect configurations on water adsorptionproperties, three structures with two missing linkers in differentarrangements (i.e., def2_90, def2_180, and def_par, see Figure1) were constructed and studied. We note that the particulardef4 structure investigated in this work was built based upon anobservation made from def2_180, where the clustering of waterwas found to be facilitated when linkers were removed alongthe ⟨110⟩ direction (see below for details). To further promotethe condensation of water, two additional linkers were thereforeremoved along this direction, resulting in a highly symmetricdef4 structure. For all of these structures with missing linkers,hydroxyl groups were attached to ensure charge neutrality. Theatomic structures for all defect structures were relaxed usingfirst-principle density functional theory (DFT) calculationsimplemented in the VASP software.31−33 Projected augmented-wave pseudopotentials34,35 were used with the generalizedgradient approximation using the Perdew−Burke−Ernzerhoffunctional.36,37 A plane-wave cutoff energy of 800 eV was usedwith a self-consistency convergence criterion of 10−4 eV and anionic relaxation criterion of 10−3 eV/Å. As the unit cell isrelatively large (i.e., ∼18 Å), single gamma-point DFTcalculations were carried out. For simplicity, all MOF structureshave the same cubic lattice with a cell length of 17.8348 Å ineach direction as experimentally determined in the previouswork.30 The coordinate information of all structures and thecorresponding unit cell formulas can be found in theSupporting Information.Gas adsorption isotherms in MOFs were calculated using

Monte Carlo within the grand canonical ensemble (i.e., GCMCsimulations) as implemented in the RASPA package.38 Theinteratomic interactions are described with 6-12 Lennard-Jones(L-J) and long-range Coulomb interactions. The L-J potentialswere truncated and shifted to zero at a cutoff distance of 12 Å,whereas the long-range Coulomb interactions were computedusing the Ewald summation technique.39 The L-J parametersfor the framework atoms were modeled using the DREIDINGforce field40 except for the zirconium atoms whose parameterswere instead from the universal force field (UFF).41 The partialcharges for the MOF atoms were determined by the REPEATalgorithm42 using electrostatic potentials obtained from theaforementioned first-principle DFT calculations. Water mole-cules were described by the TIP4P-EW model.43 In thecalculations for each structure, a supercell comprising 2 × 2 × 2unit cells was used, ensuring the simulation box to be of at leasttwice the cutoff radius along each crystal direction. To computeP/P0 in all the water isotherms presented in this study, thesaturation pressure, P0, is set to be 3750 Pa at roomtemperature,44 according to the computed vapor pressureusing the TIP4P-Ew model. The GCMC calculations werecarried out for a pressure range of 1−3750 Pa at 298 K. NVTcalculations (Monte Carlo simulations in the canonicalensemble) were also performed to investigate the interactionenergies and water adsorption configurations of the system at agiven adsorption loading. For these NVT simulations, initialconfigurations were adopted from snapshots of GCMCsimulations.

■ RESULTS AND DISCUSSIONThe computed water adsorption isotherms for perfect MOF-801 and MOF-801 with defects, along with the experimentaldata reported in the literature,30 are summarized in Figure 2.

Figure 1. Five MOF-801 defect structures with different defectdensities and/or spatial configurations are considered in this work.Green polyhedra represent the metal clusters, whereas missing linkersare highlighted in red. The relative position of the missing linkers isschematically illustrated in cubic boxes. The first number in thestructure name indicates how many linkers were removed per unit cell(e.g., def2: a MOF-801 structure with two missing linkers per unitcell). Additionally, the notations after the underscore refer to differentrelative configurations of the defects (i.e., par: defects areperpendicular to each other and located in different planes, 90:defects are perpendicular to each other and located in the same plane,180: defects are in parallel and located in the same plane).

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

B

http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpcc.8b00014

The calculated water isotherm in the perfect crystal shows alarge deviation from the experimental measurement. A gradualincrease in adsorption uptake starting at around 40% partialpressure was found in contrast to a sharp condensation at 6%relative pressure observed in experiments. Moreover, itsmaximum adsorption uptake value was calculated to be 26%smaller than that of the experimental value. The discrepancybetween simulation predictions and experimental measure-ments is substantially reduced when defects are introduced,demonstrating that defects in MOF-801 indeed play a crucialrole in its promising adsorption properties. As the number ofdefects increases, the maximum water adsorption uptakeincreases and the isotherm tends to shift to the left (i.e.,stronger adsorption affinity to water) as shown in Figure 2,indicating a more hydrophilic nature. This also suggests that thecharacteristic of water adsorption isotherms in MOFs may beused to probe their structural defects, in agreement with thatreported in the work of Dissegna et al.26 Increase in hydrophilicbehavior can be attributed to the interaction between watermolecules and the hydroxyl groups attached onto metal nodeswhen defects are introduced. Although all studied MOFs withdefects still appear to possess more hydrophobic characteristicswith a lower uptake as compared to the experimentallydetermined isotherm, the simulated isotherm of MOF-801with the highest density of defects (i.e., def4 that has a defectdensity of 1/6, representing four missing linkers per unit cell) isin decent agreement with the experimental one. A steepincrease in the water adsorption of the def4 sample occurs ataround 10% relative pressure and the uptake goes up to 92% ofthe experimental value at a high relative pressure. At this point,it is important to note that some uncertainties may be involvedin the comparison between experimental measurements andour simulation results. First, experimental measurements ofwater isotherms have been known to be challenging because of,for example, inconsistencies of the structures from the synthesisprocess, the possibility of slow dynamics inside the framework,and difficulties in precise control over the vapor pressure.45 As aspecific example, the isotherm measurements of wateradsorption in Mg-MOF-74, an open-metal-site MOF, reportedin the literature show a wide range of uptake values.45−48 Next,we have only considered one type of defects (i.e., missinglinkers with hydroxyl terminals), whereas other forms of defectsmay possibly exist as well. For instance, it has been reported

that structural defects may also result in unsaturated metalsites.49−51 With the presence of unsaturated metal sites, theadsorption strength of water at the low-pressure region may besubstantially enhanced.45 Additionally, no molecular potentialhas been specifically developed to date to model this particularsystem, and therefore, parameters from generic force fieldsincluding DREIDING40 and UFF41 were adopted in this study.It has been recently demonstrated that accurate force fields fordescribing gaseous adsorption in MOFs can be achieved byusing ab initio calculations.52−54 To accurately describe watermolecules adsorbed onto MOFs, interaction energies betweenwater and a variety of MOFs (such as MOF-74 and CuBTC)calculated by DFT have been used as references to parameter-ize potential parameters.45,55−57 However, such a detailedpotential development is out of the scope of this work and canbe an important subject of future studies. Finally, the nature ofwater simulations in porous materials requires a considerableamount of time (i.e., calculations take several months), and it istherefore difficult to ensure that global equilibrium has beenachieved. A recent study demonstrates that advanced MonteCarlo schemes such as continuous fractional component MonteCarlo58 can notably speed up the calculations for wateradsorption isotherms, which may potentially help address suchdifficulty.59 Overall, considering all the aforementioneduncertainties, the qualitative agreement between simulationsand experiments achieved herein can be considered quite good.For those samples with fewer defects (i.e., def1 and def2), theirpredicted isotherms exhibit some minor differences. Among thedef2 samples, def2_180 shows the highest uptake, whereasdef2_par and def2_90 samples show a comparable amount ofuptake. The effects of density and spatial configuration ofdefects on adsorption thermodynamics are discussed in greaterdetail below.The increased hydrophilicity with the presence of defects can

be primarily attributed to the increased Coulomb interactionfrom the hydroxyl terminal group at the defect sites. Figure 3compares the host−adsorbate (denoted as H−A between theadsorbed water molecules and the framework) interactionenergy of the perfect MOF-801 and def1 structures as afunction of adsorption uptake. For the perfect structure, the vander Waals (vdW) and Coulomb contributions of the H−Ainteraction are similar and remain relatively constant at uptakes

Figure 2. Water adsorption isotherms of perfect MOF-801 and defectstructures. The solid lines represent the results obtained from theGCMC calculations, and the experimental measurement reported inthe literature by Furukawa et al.30 is plotted with a dashed line.

Figure 3. vdW and Coulomb contributions to the H−A energy ofperfect and def1 structures. The inset displays the total H−A energy ofthese two systems. In this figure, a positive energy value represents anattractive interaction between the host (MOFs) and adsorbate (water)molecules.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

C

http://dx.doi.org/10.1021/acs.jpcc.8b00014

ranging from 0 to 25 wt %. With the introduction of defects,the Coulomb contribution to the H−A interaction energynotably increases, in particular in the low-loading region (i.e.,becomes more favorable) with a slight trade-off in the vdWinteraction. At a loading of less than 2 wt %, the H−A Coulomband total interaction energies in def1 were increased by morethan 3- and 2-fold, respectively, compared to that in perfectMOF-801. This enhanced interaction upon introducingstructural defects can provide a stronger adsorption affinity,which has been also reflected in the computed adsorptionisotherms. When the defect sites are saturated with increasedloading, the H−A interaction energy (i.e., mainly from theCoulomb contribution) begins to drop and remains fairlyconsistent with that in the perfect structure in the high-loadingregion. The H−A interaction energy in the high-loading regionis, however, still more favorable in def1, given a portion of theadsorbed molecules can still interact stronger with theframework defects. Similar observations can be seen for otherdefect structures (see Supporting Information, Figure S1).The maximum water loading in MOFs is directly related to

their performance in water adsorptive applications (e.g., ahigher saturation uptake would enable a large energy density ofwater adsorptive heat pumps).16 To better understand theunderlying mechanism behind the varying maximum loadingsfor different structures shown in Figure 2, a structural analysiswas performed. Figure 4a,b shows, respectively, the water-accessible volume ratio and surface area for the perfect anddefective structures of MOF-801. The water-accessible volumeratio, shown in Figure 4a, was determined by investigatingdetailed water adsorption configurations in fully loaded MOFstructures. Specifically, atomic configurations obtained fromGCMC calculations at ∼0.93 P/P0 were used as starting pointsfor NVT simulations. Approximately 8000 uncorrelatedadsorption configurations were obtained from NVT simulationsand superimposed. We divided the unit cell into 18 × 18 × 18voxels (i.e., the dimension of each voxel is approximately 1 × 1× 1 Å3) and analyzed if each voxel can be accessed by theoxygen atom of water molecules from the superimposed ∼8000configurations. The total water accessible volume ratio can thenbe calculated for each MOF structure. Using this wateraccessible volume is expected to provide an accurate

representation for the available space in structures that isdirectly accessible to water molecules. The surface area wascomputed by using nitrogen as a probe molecule togeometrically roll over the internal surface.60

Interestingly, although the accessible free volume has beengenerally regarded to correlate well with the maximumadsorption uptake (or the uptake in a high-pressureregion),61,62 the water-accessible volume ratio shown in Figure4a does not correspond to the maximum loading values foundin the adsorption isotherm results as given in Figure 2. Themaximum loading increases as per < def1 < def2_par = def2_90< def2_180 < def4, whereas the accessible volume insteadfollows the sequence of def1 < per < def2_180 < def2_90 <def2_par < def4. The accessible volume of def1 is found to besurprisingly smaller than that of perfect MOF-801 as one wouldintuitively anticipate that structures with defects would offer alarger available space for adsorbates. Additionally, a notablevariation in the accessible volume was found in the def2samples, although they have the same defect density. Theseunexpected results may be attributed to the inherently stressedstructure of MOF-801. Because of the asymmetric organicligands, an internal stress is present inside the framework andthe metal clusters are slightly tilted. When a linker is removed,the force balance is disrupted, which consequently leads to adistortion of the structure (e.g., the position of metal nodes isshifted). A schematic representation of this effect can be seen inthe Supporting Information, Figure S2. As a result, the removalof linkers results in a counterintuitive change in the porevolume. Similarly, it was found that the surface area, shown inFigure 4b, was also unable to explain the trend in the maximumadsorbed uptake. The surface area displays a general increasewith the increased number of defects. Removing linkers willreduce the surface area contribution from the linkers, but anadditional surface exposure of the metallic nodes which wereoriginally covered by the linkers will be gained. For longerlinkers, the loss of surface area from the organic linkers is morelikely to outweigh the additional exposure of the metallic nodes.However, in the case of MOF-801, the linkers are small enoughthat the exposure of the metallic nodes is greater. For structuresof the same defect density, a relatively small variation betweenthe three studied def2 samples was observed and this can be

Figure 4. Structural and energetic characteristics of the perfect MOF-801 framework and defect structures. (a,b) show, respectively, the wateraccessible volume ratios and surface areas of MOF-801. (c,d) display the density of water and the total interaction energy of water molecules,respectively, adsorbed in MOF-801 at a relative pressure of ∼93% (3500 Pa). In (d), a positive energy value represents an attractive interaction.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

D

http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpcc.8b00014

again attributed to the structural distortions. We note that thechoice of using N2 as the probe molecule would also allow theabove-reported geometrical surface areas to be compared withfuture experimental measurements, offering a potential meansto characterize the synthesized samples that possibly possessdifferent defect densities and/or spatial configurations.Assessing the surface area of microporous MOFs using theBET method with N2 has been the state-of-the-art approach,which also generally yields consistent surface areas as comparedto that determined geometrically by rolling a probe moleculeover the internal surface.63−65

For water adsorption in materials, as discussed above, thesetwo commonly used geometrical features are unable to predictadsorption properties. To understand the variation in thesaturated loading, we have therefore further computed thedensity of the adsorbed water phase, shown in Figure 4c, by thetotal water-accessible volume and the maximum loading at a∼93% relative pressure for each material. Surprisingly, thedensity of adsorbed water in different materials was found todiffer by as much as 20%. The corresponding total interactionenergies of adsorbed water molecules were also computed asgiven in Figure 4d. Although comparing the density of theadsorbed water (Figure 4c) to its total interaction energy(Figure 4d), a strong positive correlation was observed,indicating that the enhancement in the interaction energy ofwater molecules due to the introduction of defects has led to amuch denser water phase. With such energetic informationtogether with the geometric characteristics at our disposal, thetrend in the saturation adsorption update can be now well-explained. For instance, although the structural distortion ofdef1 has led to a smaller total available water-accessible volumecompared to the perfect crystal, its enhanced interactionenergies with a denser adsorbed phase resulted in an overallhigher maximum uptake. Our results suggest that anoptimization of the surface chemistry providing a betterstabilization to the adsorbed water phase may be a key toenhance the adsorption capacity. At this point, it is important toalso discuss the profound effects of the spatial defectconfigurations on their water adsorption characteristics. Fromthe adsorption energies of water in def2 structures withdifferent arrangements, the def2_180 shows the largest totalinteraction whereas def2_par and def2_90 possess lower andrelatively similar total interaction energies. We found that withthe specific spatial defect arrangement in def2_180 (same indef4), large and continuous hydrophilic defect channels alongthe [110] direction can be formed. Such hydrophilic channels

provide strong interactions with water molecules as well asfacilitate the clustering of water molecules, creating a morecondensed phase compared to other defect configurations. Aschematic presentation of the hydrophilic channels can befound in Figure S3 of the Supporting Information.Figure 5 shows the interaction energies of water adsorbed in

each structure as a function of water uptake. The totalinteraction energies are decomposed into two components: H−A and adsorbate−adsorbate (denoted as A−A, interactionsbetween water and water molecules). In general, as mentionedpreviously, all defect samples show a notably more favorableH−A energy compared to the perfect structure at a low loading(i.e., low-pressure region), indicating strong interactions fromthe more hydrophilic nature of the structures with defects.Among all samples, def2_180 offers the strongest interactionenergy between the host material and the adsorbate moleculefollowed by def4. At a low pressure, one would expect that thedefect structures with stronger H−A interaction would simplyexhibit greater adsorption.66,67 Interestingly, comparing theuptake at low pressures from the isotherms and the interactionenergy results, it can be seen that the H−A energy cannotexplain fully the observed adsorptive behavior. This may beattributed to the fact that, even in the low-pressure region, theA−A interaction also contributes significantly to the adsorptionbehavior. As shown in Figure 5, the A−A interaction energy indef2_par at a low pressure is more favorable than that in, forinstance, def2_180. A−A interaction energies are primarilydetermined by the relative adsorption configurations betweenadjacent adsorbed molecules, depending on a variety of factorsincluding, but not limited to, surface chemistry and poreconfinement. A detailed investigation to understand theadsorption configurations of water in structures with defectsand their configurational changes upon adsorption will be animportant subject of future study.Finally, the evolution of the water adsorption in MOF-801 as

a function of relative pressures was explored at an atomic levelwith the water density contour plots for def1 and def4structures shown in Figure 6. The density of water molecules isexamined by dividing the MOF structure into four areas alongthe z-axis (i.e., crystallographic c-direction). The thickness ofeach cross-section is 4.46 Å, 1/4 of the lattice dimension of theunit cell. In the def1 structure, at a low relative pressure of 0−27%, water molecules as expected first get adsorbed near thedefect sites because of the strong H−A interactions, forminglocal water clusters around the hydrogenated oxygen in themetal cluster. With the local water clusters formed near the

Figure 5. (a) H−A energy and the (b) A−A interaction energy as a function of loading (wt %). In this figure, a positive energy value represents anattractive interaction.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

E

http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpcc.8b00014

defects, they further facilitate water adsorption at a lowerrelative pressure and lead to a sharper increase in uptake at alower relative pressure as compared to that in the perfect crystal(see Supporting Information Figure S4 for the evolution ofwater adsorption in the perfect MOF-801 structure). Watermolecules are preferentially adsorbed in tetrahedral channelsalong the ⟨110⟩ direction, and not all channels have equal waterdensities. This unequal water adsorption can be attributed tothe heterogeneous energy surface because of (a) the presenceof defects and (b) the position of the hydroxyl groups in themetal clusters (i.e., Zr6O4(OH)4). When the adsorptiongradually reaches full saturation, the remaining free porevolumes become filled. Similarly, for the def4 case, in the low-pressure region below 10% relative pressure, water moleculesare also adsorbed first near the defect sites. However, the pores,specifically the preferential tetrahedral channels along the⟨110⟩, are much more quickly filled up, and the condensationstarts to occur at a lower relative pressure of 10%. In the def4structure, all defects are placed along the preferentialadsorption direction (i.e., ⟨110⟩ direction, see Figure S3 ofthe Supporting Information and the discussion above),resulting in large and continuous hydrophilic channels toallow a more uniform water adsorption along the defect sitesand facilitate a rapid water uptake. It should be noted again thatthere exists a collective effect for the water adsorption in def4 asboth H−A and A−A interaction energies contribute substan-

tially to the adsorption. Same as that in def1, all pores areeventually filled up at a higher pressure.

■ CONCLUSIONSThe water adsorption properties of MOF-801 were investigatedin this study. Perfect MOF-801 and MOF-801 structures withdifferent defect configurations were examined to gain acomprehensive understanding of how the density and spatialarrangement of defects affect the water adsorption properties.Our work has shown that the experimental adsorptive behaviorscan be explained through the presence of defects with the actualstructure believed to possess a high defect density. With theincreased number of missing-linker defects, MOF structurespossess more hydrophilic characteristics (i.e., stronger initialadsorption strength and steep uptake occurring at a lowerpressure). This can be mainly attributed to the increased H−ACoulomb interactions inside the defect structures. It was alsofound that the interaction energies of the adsorbed water insideconfinements play an important role in determining theirsaturation adsorption uptake. Although the total free porevolume that is available to adsorbates has been generallyregarded to correlate with the saturation uptake of adsorbates,varying degrees of energy stabilization for adsorbed water canresult in a notable difference in its density (i.e., a difference ashigh as 20% was observed). This result suggests that, forapplications such as water harvesting and heat pumps usingMOFs, optimizing the surface chemistry by controlling defectsvia different synthesis approaches and/or tuning ligands maylargely improve their storage performance. Additionally, ourcalculations indicate that, unlike other gases such as CO2 andN2, the adsorptive behavior of water in the low-pressure regiondepends not only on the H−A interactions but also on the A−A interactions. Finally, a preferential water condensation wasidentified in the ⟨110⟩ directions of MOF-801, along thetetrahedral pore sites. When defects exist in this direction (e.g.,def4 and def2_180), stronger adsorptive behavior can berealized. This highlights the importance of the spatialconfigurations of defects in the water adsorption properties.It should be noted, however, that the spatial arrangement ofdefects and the defect types were not exhaustively explored inthis study (e.g., only one defect arrangement was studied forthe defect density of 1/6 and only missing-linker defects wereconsidered), and an extensive investigation of these effects canbe an important subject of future studies. Overall, this work hasillustrated the role of defects and offers a microscopic picture oftheir effects on the adsorption properties of water in MOFs.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b00014.

Additional figures referred in the main article and thecoordinate files of all studied structures including thedetailed energy contributions to the interactions betweenwater molecules and MOF frameworks in all studiedstructures, schematic illustrations of the structuralchanges due to the missing linkers and the hydrophilicchannels observed in def2_180 and def4 structures, waterdensity plots of the perfect MOF-801 structure atdifferent loadings, and structure information (PDF)

Figure 6. Water density plots in def1 and def4 structures at differentadsorption loadings (wt %). The cross-sectional cut and the layout ofthe density contours plots are schematically shown in the upper panelof the figure. The position of missing linkers in def1 and def4structures is also highlighted in red dashed lines. The water densityplots in the perfect MOF-801 structure can be seen in the SupportingInformation Figure S4.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

F

http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acs.jpcc.8b00014http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.8b00014/suppl_file/jp8b00014_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpcc.8b00014

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: lin.2645@osu.edu (L.-C.L.).*E-mail: jcg@mit.edu (J.C.G.).ORCIDLi-Chiang Lin: 0000-0002-2821-9501Author ContributionsThis study was developed and completed through contributionsby all authors, and all authors have given approval to the finalversion of the manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully thank the Ohio Supercomputer Center(OSC)68 for providing computational resources to carry outthis work. This research also used resources of the NationalEnergy Research Scientific Computing Center, a DOE Office ofScience User Facility supported by the Office of Science of theU.S. Department of Energy under contract no. DE-AC02-05CH11231, as well as of the Extreme Science and EngineeringDiscovery Environment (XSEDE), which is supported byNational Science Foundation grant number ACI-1548562.

■ ABBREVIATIONSH−A, host−adsorbate; A−A, adsorbate−adsorbate

■ REFERENCES(1) Li, H. L.; Eddaoudi, M. M.; O’Keeffe, M.; Yaghi, O. M. Designand Synthesis of an Exceptionally Stable and Highly Porous Metal-Organic Framework. Nature 1999, 402, 276−279.(2) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.;Hupp, J. T. Metal-Organic Framework Materials as Catalysts. Chem.Soc. Rev. 2009, 38, 1450−1459.(3) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne,R. P.; Hupp, J. T. Metal−Organic Framework Materials as ChemicalSensors. Chem. Rev. 2012, 112, 1105−1125.(4) Stock, N.; Biswas, S. Synthesis of Metal-Organic Frameworks(MOFs): Routes to Various MOF Topologies, Morphologies, andComposites. Chem. Rev. 2012, 112, 933−969.(5) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Selective Gas Adsorption andSeparation in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38,1477−1504.(6) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. TheChemistry and Applications of Metal-Organic Frameworks. Science2013, 341, 1230444.(7) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C.E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. Ö.; Hupp,J. T. Metal−Organic Framework Materials with Ultrahigh SurfaceAreas: Is the Sky the Limit? J. Am. Chem. Soc. 2012, 134, 15016−15021.(8) Huck, J. M.; Lin, L.-C.; Berger, A. H.; Shahrak, M. N.; Martin, R.L.; Bhown, A. S.; Haranczyk, M.; Reuter, K.; Smit, B. EvaluatingDifferent Classes of Porous Materials for Carbon Capture. EnergyEnviron. Sci. 2014, 7, 4132−4146.(9) Kim, J.; Lin, L.-C.; Martin, R. L.; Swisher, J. A.; Haranczyk, M.;Smit, B. Large-Scale Computational Screening of Zeolites for Ethane/Ethene Separation. Langmuir 2012, 28, 11914−11919.(10) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon DioxideCapture: Prospects for New Materials. Angew. Chem., Int. Ed. 2010, 49,6058−6082.(11) Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.;Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N. WaterHarvesting from Air with Metal-Organic Frameworks Powered byNatural Sunlight. Science 2017, 356, 430−434.

(12) Henninger, S. K.; Habib, H. A.; Janiak, C. MOFs as Adsorbentsfor Low Temperature Heating and Cooling Applications. J. Am. Chem.Soc. 2009, 131, 2776−2777.(13) Ehrenmann, J.; Henninger, S. K.; Janiak, C. Water AdsorptionCharacteristics of MIL-101 for Heat-Transformation Applications ofMOFs. Eur. J. Inorg. Chem. 2011, 471−474.(14) Kim, H.; Cho, H. J.; Narayanan, S.; Yang, S.; Furukawa, H.;Schiffres, S.; Li, X.; Zhang, Y.-B.; Jiang, J.; Yaghi, O. M.; et al.Characterization of Adsorption Enthalpy of Novel Water-StableZeolites and Metal-Organic Frameworks. Sci. Rep. 2016, 6, 19097.(15) de Lange, M. F.; van Velzen, B. L.; Ottevanger, C. P.; Verouden,K. J. F. M.; Lin, L.-C.; Vlugt, T. J. H.; Gascon, J.; Kapteijn, F. Metal−Organic Frameworks in Adsorption-Driven Heat Pumps: ThePotential of Alcohols as Working Fluids. Langmuir 2015, 31,12783−12796.(16) de Lange, M. F.; Verouden, K. J. F. M.; Vlugt, T. J. H.; Gascon,J.; Kapteijn, F. Adsorption-Driven Heat Pumps: The Potential ofMetal−Organic Frameworks. Chem. Rev. 2015, 115, 12205−12250.(17) Fang, Z.; Bueken, B.; De Vos, D. E.; Fischer, R. A. Defect-Engineered Metal−Organic Frameworks. Angew. Chem., Int. Ed. 2015,54, 7234−7254.(18) Bai, Y.; Dou, Y.; Xie, L.-H.; Rutledge, W.; Li, J.-R.; Zhou, H.-C.Zr-Based Metal-Organic Frameworks: Design, Synthesis, Structure,and Applications. Chem. Soc. Rev. 2016, 45, 2327−2367.(19) Chong, S.; Thiele, G.; Kim, J. Excavating Hidden AdsorptionSites in Metal-Organic Frameworks Using Rational Defect Engineer-ing. Nat. Commun. 2017, 8, 1539.(20) Valenzano, L.; Civalleri, B.; Chavan, S.; Bordiga, S.; Nilsen, M.H.; Jakobsen, S.; Lillerud, K. P.; Lamberti, C. Disclosing the ComplexStructure of UiO-66 Metal Organic Framework: A SynergicCombination of Experiment and Theory. Chem. Mater. 2011, 23,1700−1718.(21) Cliffe, M. J.; Wan, W.; Zou, X.; Chater, P. A.; Kleppe, A. K.;Tucker, M. G.; Wilhelm, H.; Funnell, N. P.; Coudert, F.-X.; Goodwin,A. L. Correlated Defect Nanoregions in a Metal−organic Framework.Nat. Commun. 2014, 5, 4176.(22) Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.;Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned toPerfection: Ironing Out the Defects in Metal−Organic FrameworkUiO-66. Chem. Mater. 2014, 26, 4068−4071.(23) Taylor, J. M.; Dekura, S.; Ikeda, R.; Kitagawa, H. Defect ControlTo Enhance Proton Conductivity in a Metal−Organic Framework.Chem. Mater. 2015, 27, 2286−2289.(24) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.;Yildirim, T.; Zhou, W. Unusual and Highly Tunable Missing-LinkerDefects in Zirconium Metal−Organic Framework UiO-66 and TheirImportant Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135,10525−10532.(25) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building BrickForming Metal Organic Frameworks with Exceptional Stability. J. Am.Chem. Soc. 2008, 130, 13850−13851.(26) Dissegna, S.; Hardian, R.; Epp, K.; Kieslich, G.; Coulet, M.-V.;Llewellyn, P.; Fischer, R. A. Using Water Adsorption Measurements toAccess the Chemistry of Defects in the Metal-Organic FrameworkUiO-66. CrystEngComm 2017, 19, 4137−4141.(27) Ghosh, P.; Coloń, Y. J.; Snurr, R. Q. Water Adsorption in UiO-66: The Importance of Defects. Chem. Commun. 2014, 50, 11329−11331.(28) Liang, W.; Coghlan, C. J.; Ragon, F.; Rubio-Martinez, M.;D’Alessandro, D. M.; Babarao, R. Defect Engineering of UiO-66 forCO2 and H2O Uptakea Combined Experimental and SimulationStudy. Dalton Trans. 2016, 45, 4496−4500.(29) Wißmann, G.; Schaate, A.; Lilienthal, S.; Bremer, I.; Schneider,A. M.; Behrens, P. Modulated Synthesis of Zr-Fumarate MOF.Microporous Mesoporous Mater. 2012, 152, 64−70.(30) Furukawa, H.; Gańdara, F.; Zhang, Y.-B.; Jiang, J.; Queen, W. L.;Hudson, M. R.; Yaghi, O. M. Water Adsorption in Porous Metal−

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

G

mailto:lin.2645@osu.edumailto:jcg@mit.eduhttp://orcid.org/0000-0002-2821-9501http://dx.doi.org/10.1021/acs.jpcc.8b00014

Organic Frameworks and Related Materials. J. Am. Chem. Soc. 2014,136, 4369−4381.(31) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for AbInitio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys.Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186.(32) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for LiquidMetals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561.(33) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulationof the Liquid-Metal−Amorphous-Semiconductor Transition inGermanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49,14251−14269.(34) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B:Condens. Matter Mater. Phys. 1994, 50, 17953−17979.(35) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to theProjector Augmented-Wave Method. Phys. Rev. B: Condens. MatterMater. Phys. 1999, 59, 1758−1775.(36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized GradientApproximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.(37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized GradientApproximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396.(38) Dubbeldam, D.; Calero, S.; Ellis, D. E.; Snurr, R. Q. RASPA:Molecular Simulation Software for Adsorption and Diffusion inFlexible Nanoporous Materials. Mol. Simul. 2016, 42, 81−101.(39) Frenkel, D.; Smit, B. Understanding Molecular Simulation, 2nded.; Academic Press: San Diego, 2002.(40) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING: AGeneric Force Field for Molecular Simulations. J. Phys. Chem. 1990,94, 8897−8909.(41) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.;Skiff, W. M. UFF, a Full Periodic Table Force Field for MolecularMechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc.1992, 114, 10024−10035.(42) Campaña,́ C.; Mussard, B.; Woo, T. K. Electrostatic PotentialDerived Atomic Charges for Periodic Systems Using a Modified ErrorFunctional. J. Chem. Theory Comput. 2009, 5, 2866−2878.(43) Horn, H. W.; Swope, W. C.; Pitera, J. W.; Madura, J. D.; Dick,T. J.; Hura, G. L.; Head-Gordon, T. Development of an ImprovedFour-Site Water Model for Biomolecular Simulations: TIP4P-Ew. J.Chem. Phys. 2004, 120, 9665−9678.(44) Horn, H. W.; Swope, W. C.; Pitera, J. W. Characterization of theTIP4P-Ew Water Model: Vapor Pressure and Boiling Point. J. Chem.Phys. 2005, 123, 194504.(45) Lin, L.-C.; Lee, K.; Gagliardi, L.; Neaton, J. B.; Smit, B. Force-Field Development from Electronic Structure Calculations withPeriodic Boundary Conditions: Applications to Gaseous Adsorptionand Transport in Metal-Organic Frameworks. J. Chem. Theory Comput.2014, 10, 1477−1488.(46) Yang, D.-A.; Cho, H.-Y.; Kim, J.; Yang, S.-T.; Ahn, W.-S. CO2Capture and Conversion Using Mg-MOF-74 Prepared by aSonochemical Method. Energy Environ. Sci. 2012, 5, 6465−6473.(47) Schoenecker, P. M.; Carson, C. G.; Jasuja, H.; Flemming, C. J. J.;Walton, K. S. Effect of Water Adsorption on Retention of Structureand Surface Area of Metal−Organic Frameworks. Ind. Eng. Chem. Res.2012, 51, 6513−6519.(48) Grant Glover, T.; Peterson, G. W.; Schindler, B. J.; Britt, D.;Yaghi, O. MOF-74 Building Unit Has a Direct Impact on Toxic GasAdsorption. Chem. Eng. Sci. 2011, 66, 163−170.(49) Jiang, Z.-R.; Wang, H.; Hu, Y.; Lu, J.; Jiang, H.-L. Polar Groupand Defect Engineering in a Metal−Organic Framework: SynergisticPromotion of Carbon Dioxide Sorption and Conversion. ChemSu-sChem 2015, 8, 878−885.(50) Vermoortele, F.; Bueken, B.; Le Bars, G.; Van de Voorde, B.;Vandichel, M.; Houthoofd, K.; Vimont, A.; Daturi, M.; Waroquier, M.;Van Speybroeck, V.; et al. Synthesis Modulation as a Tool To Increasethe Catalytic Activity of Metal−Organic Frameworks: The UniqueCase of UiO-66(Zr). J. Am. Chem. Soc. 2013, 135, 11465−11468.(51) Vandichel, M.; Hajek, J.; Vermoortele, F.; Waroquier, M.; DeVos, D. E.; Van Speybroeck, V. Active Site Engineering in UiO-66

Type Metal-Organic Frameworks by Intentional Creation of Defects:A Theoretical Rationalization. CrystEngComm 2015, 17, 395−406.(52) Dzubak, A. L.; Lin, L.-C.; Kim, J.; Swisher, J. A.; Poloni, R.;Maximoff, S. N.; Smit, B.; Gagliardi, L. Ab Initio Carbon Capture inOpen-Site Metal−organic Frameworks. Nat. Chem. 2012, 4, 810−816.(53) Fang, H.; Kamakoti, P.; Zang, J.; Cundy, S.; Paur, C.;Ravikovitch, P. I.; Sholl, D. S. Prediction of CO2 AdsorptionProperties in Zeolites Using Force Fields Derived from PeriodicDispersion-Corrected DFT Calculations. J. Phys. Chem. C 2012, 116,10692−10701.(54) Borycz, J.; Lin, L.-C.; Bloch, E. D.; Kim, J.; Dzubak, A. L.;Maurice, R.; Semrouni, D.; Lee, K.; Smit, B.; Gagliardi, L. CO2Adsorption in Fe2(dobdc): A Classical Force Field Parameterizedfrom Quantum Mechanical Calculations. J. Phys. Chem. C 2014, 118,12230−12240.(55) Rudenko, A. N.; Bendt, S.; Keil, F. J. Multiscale Modeling ofWater in Mg-MOF-74: From Electronic Structure Calculations toAdsorption Isotherms. J. Phys. Chem. C 2014, 118, 16218−16227.(56) Zang, J.; Nair, S.; Sholl, D. S. Prediction of Water Adsorption inCopper-Based Metal−Organic Frameworks Using Force FieldsDerived from Dispersion-Corrected DFT Calculations. J. Phys. Chem.C 2013, 117, 7519−7525.(57) Mercado, R.; Vlaisavljevich, B.; Lin, L.-C.; Lee, K.; Lee, Y.;Mason, J. A.; Xiao, D. J.; Gonzalez, M. I.; Kapelewski, M. T.; Neaton, J.B.; et al. Force Field Development from Periodic Density FunctionalTheory Calculations for Gas Separation Applications Using Metal−Organic Frameworks. J. Phys. Chem. C 2016, 120, 12590−12604.(58) Shi, W.; Maginn, E. J. Continuous Fractional ComponentMonte Carlo: An Adaptive Biasing Method for Open SystemAtomistic Simulations. J. Chem. Theory Comput. 2007, 3, 1451−1463.(59) Zhang, H.; Snurr, R. Q. Computational Study of WaterAdsorption in the Hydrophobic Metal−Organic Framework ZIF-8:Adsorption Mechanism and Acceleration of the Simulations. J. Phys.Chem. C 2017, 121, 24000−24010.(60) Düren, T.; Millange, F.; Feŕey, G.; Walton, K. S.; Snurr, R. Q.Calculating Geometric Surface Areas as a Characterization Tool forMetal−Organic Frameworks. J. Phys. Chem. C 2007, 111, 15350−15356.(61) Wilmer, C. E.; Leaf, M.; Lee, C. Y.; Farha, O. K.; Hauser, B. G.;Hupp, J. T.; Snurr, R. Q. Large-Scale Screening of HypotheticalMetal−organic Frameworks. Nat. Chem. 2011, 4, 83−89.(62) Martin, R. L.; Lin, L.-C.; Jariwala, K.; Smit, B.; Haranczyk, M.Mail-Order Metal−Organic Frameworks (MOFs): Designing Iso-reticular MOF-5 Analogues Comprising Commercially AvailableOrganic Molecules. J. Phys. Chem. C 2013, 117, 12159−12167.(63) Walton, K. S.; Snurr, R. Q. Applicability of the BET Method forDetermining Surface Areas of Microporous Metal-Organic Frame-works. J. Am. Chem. Soc. 2007, 129, 8552−8556.(64) Goḿez-Gualdroń, D. A.; Moghadam, P. Z.; Hupp, J. T.; Farha,O. K.; Snurr, R. Q. Application of Consistency Criteria To CalculateBET Areas of Micro- And Mesoporous Metal−Organic Frameworks. J.Am. Chem. Soc. 2016, 138, 215−224.(65) de Lange, M. F.; Lin, L.-C.; Gascon, J.; Vlugt, T. J. H.; Kapteijn,F. Assessing the Surface Area of Porous Solids: Limitations, ProbeMolecules, and Methods. Langmuir 2016, 32, 12664−12675.(66) Lin, L.-C.; Berger, A. H.; Martin, R. L.; Kim, J.; Swisher, J. A.;Jariwala, K.; Rycroft, C. H.; Bhown, A. S.; Deem, M. W.; Haranczyk,M.; et al. In Silico Screening of Carbon-Capture Materials. Nat. Mater.2012, 11, 633−641.(67) Wilmer, C. E.; Farha, O. K.; Bae, Y.-S.; Hupp, J. T.; Snurr, R. Q.Structure−property Relationships of Porous Materials for CarbonDioxide Separation and Capture. Energy Environ. Sci. 2012, 5, 9849−9856.(68) Ohio Supercomputer Center. http://osc.edu/ark:/19495/f5s1ph73. accessed Feb 16, 2018.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b00014J. Phys. Chem. C XXXX, XXX, XXX−XXX

H

http://osc.edu/ark:/19495/f5s1ph73http://osc.edu/ark:/19495/f5s1ph73http://dx.doi.org/10.1021/acs.jpcc.8b00014